Offshoreonly.com - 502 mag wont idle

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Sounds like the IAC to me, what are the iac counts on the scan tool? If ity was bad the number (I think) should be real high. The ecu tyells the iac to open so much, if it doesn't seem to respond I'm not sure it will throw codes but thats what I would be checking. I would also be looking for a new mechanic, Smitty

I agree with the other guys, sounds like a IAC or MAP sensor. Could be unpluged or defective, the IAC is a stepper motor it will not usually trigger a code.
Oh, and fire the mechanic

The ecm has no way of knowing the actual position of the IAC. All it knows is what it is commanding the IAC to do. The ecm doesn't know if the IAC is actually moving or not. It will not throw a code for the IAC not moving or moving irradically, or if it is unplugged for that matter.
It sounds like a simple problem. You also need to check the tps reading at idle, especially since the ecm has been programmed. There is a table to calculate the tps opening and closing percents. Make sure it is zeroed at idle and 100% at wot. (check with the key on and engine not running:lolhit:)
Good luck. Give me a shout if I can help.
Eddie

Thanks for all the replies. I just called the shop foreman at the marina, and he claims they have checked everything I suggest. He cannot tell me if the IAC was pulled and cleaned, but he did tell me that the reason they had the ECU reflashed was because it was throwing a map code. He said he would have the tech check the map sensor and verify it is the correct one. I also tried to explain to him that the IAC would not throw a code, but I think he is getting tired of hearing from me seeing how it is not actually my boat.

My shop is in the Austin area. Be glad to take a look at it or offer advice. Sounds like a simple fix. I doubt it's anything internal. Is the engine in the boat now? I have the capability to reprogram the pcm if needed to get it correct, although it shouldn't need that.

Bob Lloyd
Full Throttle Marine

Quote:

Originally Posted by tx911 (Post 3328480)

My brother bought a 502 mag from someone off this site, it was complete minus vst and accessories. His boat got damaged in July and that is when he decided to upgrade the motor while the other one was out, the boat had a 330hp 7.4lx that ran great, just wanted more power.

The marina that is working on the boat ran a compression check and all cylinders where within a few pounds of 165psi. They started the motor on the ground and it ran, but will not idle on its own. They say after you get it over 1500rpm it sounds fine, it just dies at idle. They said the injectors where bad, so they where sent to a injection service place in Austin, and all injectors where within 1% of each other. then they said it was the ECU, so my brother bought another one off oso, they said it was not right either so they sent them both to merc for a reflash, motor still does the same thing. I called them today and talked to the shop foreman, he says they have checked the ignition and for vacuum leaks, and there are none, he also claims the fuel pressure has been checked and it is fine.
He did not go into detail about what all they did to check the ignition, fuel, or vacuum, and I did not want to grill him about every detail as I am not actually the paying customer. He thinks there is something mechanically wrong with the motor like a wiped out cam lobe or something, but I think he is grasping at straws. The motor has 52 or 57 hours on it according to the scan tool, and I trust the person the motor came from, and he says the motor ran great. Everything he has told me about the motor thus far has been totally above board, like the fact that the injectors in the motor where new.
Does anyone that made it through this rediculously long post have any ideas I can tell the marina to try, because the only other option at this point is for me to drive 6 hours one way and pick up the motor to fix it myself. My brother does not want the marina to tear it down for obvious reasons, and I think there is a simple awnser they are missing. Thanks for any suggestions or ideas.

I'm the brother here....I just got off the phone with the shop, and they found that the injector electronics were wired backwards. They stated that it was wired according to the Merc shop manual, and that evidently the manual is wrong. Thot sounds rather suspect to me, but oh well, it is idling correctly now. I can't imagine that every mercury published shop manual would A) be incorrect, or B) not have a technical bulleting sent to certified dealers. I may be wrong on that, but I'm certainly not paying for poor troubleshooting skills/publishing errors. Anyway, thanks for all the replies guys, and I have definitely found a different shop to take my business to in the future. Not a shameless plug here, but Bob over at Full Throttle Marine was extremely helpful over the phone, (he posted in this thread also). I'll absolutely go there from now on.

Glad they got it sorted out.

Bob

Quote:

Originally Posted by txtraveller2002 (Post 3330821)

[QUOTE=tx911;3328696]The marina says it is not throwing any codes. QUOTE
What an ugly thing to say.

Based on that explaination I would never go back to that shop for service. The injectors are batch fired, 4 at a time, and the wiring harness has very little excess wire for things to be crossed up. They are telling you a complcated sounding explaination to justify a large bill for their bad diagnostic skills. RUN. Just my opinion.

I just repaired mine for the same issue. I absolutely guarantee that it was a vacuum leak. I checked for leaks everywhere and finally just pulled everything apart, cleaned, and installed new gaskets and it idles perfect. Glad you got it running.